The present invention relates to a direction-dependent driving coupling between the rotor of a permanent-magnet synchronous motor and the working part.
It is known that permanent-magnet synchronous motors are bidirectional, i.e., at power-on the rotor can be induced equally to rotate clockwise or counterclockwise.
This depends on the arrangement of the polarities of the rotor with respect to the magnetic field that forms between the poles of the stator pack when the inductor winding is supplied with alternating current.
Although this is not a problem in the case of the actuation of centrifugal pumps with radial vanes, it becomes a considerable limitation for centrifugal pumps and for fans having a particular configuration of the vanes and therefore a single direction of rotation of the impeller.
This is one of the main problems in the field of centrifugal machines which use permanent-magnet motors, and various electrical and mechanical devices have been proposed so far.
Among these, the motor pump unit described in EP 148343 is particularly significant.
This motor pump unit has a box-like body with a permanent-magnet synchronous electric motor, the rotor whereof is arranged within a substantially cup-shaped coaxial jacket arranged in the gap.
The rotor supports, at one end, an impeller whose hub lies coaxially inside the jacket.
An intermediate sleeve, driven by the rotor, is arranged between the impeller on one side and the corresponding final section of the rotor on the other side.
The intermediate sleeve comprises at least one cam which protrudes radially outwards; a tab co-operates with the cam on the internal surface of the hub and protrudes radially at the path of the cam.
The tab is part of a flexible braking flap formed in the hub.
When the cam of the intermediate element encounters the tab of the hub, the flap flexes and rests against the jacket that contains the rotor, blocking the rotation.
The hub is also provided with at least one tooth which protrudes radially inwards and with which the cam makes contact by rotating in the opposite direction with respect to the above cited one, making it rotate and rotating the impeller rigidly with the rotor.
This motor pump requires precise calibration in the regions of contact between the cam and the tab that is part of the flexible flap in order to avoid jamming.
The choice of the material that constitutes the flexible flap and the tab is in fact linked to the choice of the material of the impeller, with which it is monolithic since it is part of the hub.
The plastic material that constitutes the impeller, despite having a certain elasticity, must of course be predominantly rigid and accordingly the necessary elasticity of the flexible flap is the result of a compromise.
The fact of using mostly rigid plastic materials also has the inevitable consequence of increasing noise caused by impacts between the components at start-up.
Another device for producing direction-dependent rotations, which in this case is not linked to the problem of permanent-magnet electric motors, is known from GB 361656, in which a driving shaft supports two diametrically mutually opposite cams which are keyed to the shaft and which in one direction of rotation interfere with respective pawls pivoted in regions which are peripheral to the cams and support respective blocks arranged close to a cylindrical wall of a part to be driven.
The cams have a circular external surface, while the blocks have an eccentric circular internal surface which is arranged close to the surfaces of the cams.
Accordingly, rotary motion in one direction causes the cams to rigidly couple to the pawls, whereas by rotating in the opposite direction they produce a rotation of the blocks with respect to the respective fulcrums and lock them against the cylindrical wall.
In this case, although the flexible elements have been replaced with pivoted elements with respect to EP 148343, there is still an absolute need for perfect calibration of the geometry of the various parts, which must have a certain rigidity.